Pathogenic microbes, molds, mildew, spores, and organic and inorganic pollutants are commonly found in the environment. Microbial control and disinfection in environmental spaces is desirable to improve health. Numerous ways have been used to in the past in an attempt to purify air and disinfect surfaces. For example, it is already known that Reactive Oxidizing Species (ROS) produced by, e.g., photocatalytic oxidation process can oxidize organic pollutants and kill microorganisms. More particularly, hydroxyl radical, hydroperoxyl radicals, chlorine and ozone, end products of the photocatalytic reaction, have been known to be capable of oxidizing organic compounds and killing microorganisms. However, there are limitations to the known methods and devices, not only due to efficacy limitation but also due to safety issues.
ROS is the term used to describe the highly activated air that results from exposure of ambient humid air to ultraviolet light. Light in the ultraviolet range emits photons at a frequency that when absorbed has sufficient energy to break chemical bonds. UV light at wavelengths of 250-255 nm is routinely used as a biocide. Light below about 181 nm, up to 182-187 nm is competitive with corona discharge in its ability to produce ozone. Ozonation and UV radiation are both being used for disinfection in community water systems. Ozone is currently being used to treat industrial wastewater and cooling towers.
Hydrogen peroxide is generally known to have antimicrobial properties and has been used in aqueous solution for disinfection and microbial control. Attempts to use hydrogen peroxide in the gas phase, however, have previously been hampered by technical hurdles to the production of Purified Hydrogen Peroxide Gas (PHPG). Vaporized aqueous solutions of hydrogen peroxide produce an aerosol of microdroplets composed of aqueous hydrogen peroxide solution. Various processes for “drying” vaporized hydrogen peroxide solutions produce, at best, a hydrated form of hydrogen peroxide. These hydrated hydrogen peroxide molecules are surrounded by water molecules bonded by electrostatic attraction and London Forces. Thus, the ability of the hydrogen peroxide molecules to directly interact with the environment by electrostatic means is greatly attenuated by the bonded molecular water, which effectively alters the fundamental electrostatic configuration of the encapsulated hydrogen peroxide molecule. Further, the lowest concentration of vaporized hydrogen peroxide that can be achieved is generally well above the 1.0 ppm OSHA workplace safety limit, making these processes unsuitable for use in occupied areas.
Photocatalysts that have been demonstrated for the destruction of organic pollutants in fluid include but are not limited to TiO2, ZnO, SnO2, WO3, CdS, ZrO2, SB2O4 and Fe2O3. Titanium dioxide is chemically stable, has a suitable bandgap for UV/Visible photoactivation, and is relatively inexpensive. Therefore, photocatalytic chemistry of titanium dioxide has been extensively studied over the last thirty years for removal of organic and inorganic compounds from contaminated air and water.
Because photocatalysts can generate hydroxyl radicals from adsorbed water when activated by ultraviolet light of sufficient energy, they show promise for use in the production of PHPG for release into the environment when applied in the gas phase. Existing applications of photocatalysis, however, have focused on the generation of a plasma containing many different reactive chemical species. Further, the majority of the chemical species in the photocatalytic plasma are reactive with hydrogen peroxide, and inhibit the production of hydrogen peroxide gas by means of reactions that destroy hydrogen peroxide. Also, any organic gases that are introduced into the plasma inhibit hydrogen peroxide production both by direct reaction with hydrogen peroxide and by the reaction of their oxidized products with hydrogen peroxide.
The photocatalytic reactor itself also limits the production of PHPG for release into the environment. Because hydrogen peroxide has greater chemical potential than oxygen to be reduced as a sacrificial oxidant, it is preferentially reduced as it moves downstream in photocatalytic reactors as rapidly as it is produced by the oxidation of water.2photons+2H2O→2OH*+2H++2e−2OH*→H2O2  OxidationH2O2+2H++2e−→2H2O  ReductionAdditionally, several side reactions generate a variety of species that become part of the photocatalytic plasma, and which inhibit the production of PHPG for release into the environment as noted above.
The wavelengths of light used to activate photocatalysts are also energetic enough to photolyze the peroxide bond in a hydrogen peroxide molecule and are also an inhibitor in the production of PHPG for release into the environment. Further, the practice of using wavelengths of light that produce ozone introduces yet another species into the photocatalytic plasma that destroys hydrogen peroxide.O3+H2O2→H2O+2O2 
In practice, photocatalytic applications have focused on the production of a plasma, often containing ozone, used to oxidize organic contaminants and microbes. Such plasmas are primarily effective within the confines of the reactor itself, by nature have limited chemical stability beyond the confines of the reactor, and actively degrade the limited amounts of hydrogen peroxide gas that they may contain. Further, because the plasma is primarily effective within the reactor itself, many designs maximize residence time to facilitate more complete oxidation of organic contaminants and microbes as they pass through the reactor. Since hydrogen peroxide has such a high potential to be reduced, the maximized residence time results in minimized hydrogen peroxide output.
Also, most applications of photocatalysis produce environmentally objectionable chemical species. First among these is ozone itself, an intentional product of many systems. Further, since organic contaminants that pass through a reactor are seldom oxidized in one exposure, multiple air exchanges are necessary to achieve full oxidation to carbon dioxide and water. As incomplete oxidation occurs, a mixture of aldehydes, alcohols, carboxylic acids, ketones, and other partially oxidized organic species is produced by the reactor. Often, photocatalytic reactors can actually increase the overall concentration of organic contaminants in the air by fractioning large organic molecules into multiple small organic molecules such as formaldehyde.
In summary, the production of PHPG for release into the environment is not achieved in the prior art. Methods of vaporizing aqueous hydrogen peroxide solutions produce, at best, hydrated forms of hydrogen peroxide. Also, though photocatalytic systems are capable of producing hydrogen peroxide, they have multiple limitations that severely inhibit PHPG production for release into the environment.